The present invention relates to the shape of an electrode included in a main lens of an electron gun for color picture tube.
FIG. 2 is a longitudinal section view of a color picture tube having an electron gun of a conventional structure. A phosphor screen 3 alternately coated with three color phosphors in a stripe form is supported on the inner wall of a face plate portion 2 of a glass envelope 1. Respective central axes 15, 16 and 17 of cathodes 6, 7 and 8 coincide with central axes of apertures of a G1 cathode 9, a G2 electrode 10, a focusing electrode 11 constituting a main lens and shield cup 13, corresponding to respective cathodes and are so arranged on a common plane as to be parallel each other. Whereas the central axis of an aperture located at the center of an accelerating electrode 12, which is the other of electrodes constituting the main lens, coincides with the above described central axis, central axes 18 and 19 of both outer apertures do not coincide with their respective central axes 15 and 17 but are slightly displaced from to the outside. Three electron beams emitted from respective cathodes are applied to the main lens along the central axes 15, 16 and 17. The focusing electrode 11 is supplied with focusing voltage of approximately 5 to 10 kV, and the accelerating electrode 12 is supplied with accelerating voltage of approximately 20 to 30 kV. The accelerating electrode 12 has the same potential as that of the shield cup 13 and a conductive layer 5 disposed inside of the glass envelope.
Since the apertures located at respective centers of the focusing electrode and the accelerating electrode are coaxial, the main lens formed at the center becomes axisymmetric. After the central beam has been converged by the main lens, it goes straight ahead on a trajectory along the axis. On the other hand, outer apertures of the focusing electrode and the accelerating electrode have axes displaced each other. At the outer sides, therefore, main lenses which are not axisymmetric are formed. In a diverging lens region formed in the acceleration electrode side of the main lens region, therefore, each of the outer beams passes through a portion located nearer to the central beam with respect to the center axis of the lens and undergoes converging force directed toward the central beam concurrently with the focusing effect applied thereto by the main lens. Three electron beams thus form an image on a shadow mask 4, and at the same time converge so as to overlap each other. Such action of converging beams is referred to as static convergence (hereafter abbreviated to STC). Further, respective electron beams undergo color selection in the shadow mask. Only components which excite phosphors having colors corresponding to respective beams to emit light pass through apertures of the shadow mask and arrive at the phosphor screen. In order to scan the phosphor screen with the electron beam, an external magnetic deflection yoke 14 is disposed.
When an in-line electron gun having three electron beam paths disposed on one horizontal plane is combined with a so-called self convergence deflection yoke forming special nonuniform magnetic field distribution, it is known that if STC is established at the center of the screen, convergence is obtained over the entire remaining regions of the screen. In a typical self convergence deflection yoke, however, deflection defocusing is large because of nonuniformity of the magnetic field, resulting in a problem of lowered resolution at peripheral parts of the screen. FIG. 3 schematically shows deformation of an electron beam spot caused by deflection defocusing. At peripheral parts of the screen, the high luminance portion (core) of the electron beam indicated by the shaded region expands in the horizontal direction, while the low luminance portion (halo) expands in the vertical direction.
One means for solving this problem is shown in JP-A-61-99249. FIGS. 4A to 4C show an example of structure of an electron gun according to this prior art. The focusing electrode is bisected into a first member 114 and a second member 115 in a direction extending from the cathode to the phosphor screen. 0n an end face of the first member 114 opposed to the second member 115, slits elongated in the longitudinal direction are fomed as shown in FIG. 4B. On an end face of the second member 115 opposed to the first member, slitlike apertures elongated in the horizontal direction are formed as shown in FIG. 4C and are supplied with voltage, which changes dynamically in synchronism with the deflection current supplied to the deflection yoke, i.e., dynamic voltage superimposed over the focusing voltage Vf. When the amount of deflection is large, the potential difference between the first member 114 and the second member 115 becomes large. Therefore, refractive power of a quadrupole-lens formed by the slits becomes high, and large astigmatism is caused in the electron beam spot. If the potential of the second member 115 is higher than the potential of the first member and the third member, astigmatism caused in the electron beam has an effect of elongating the core in the vertical direction and elongating the halo in the horizontal direction. Therefore, it becomes possible to cancel astigmatism caused by electron beam deflection as shown in FIG. 3 and enhance the resolution at peripheral parts of the screen. On the other hand, the resolution is not deteriorated when the electron beam is not deflected. Because such a condition that astigmatism is not caused at the central parts of the screen can be established by eliminating the potential difference between the first member and the second member to prevent a nonsymmetrical lens from being formed.
In color picture tubes, the distance from the main lens to the peripheral parts of the screen is larger than the distance from the main lens to the central part of the screen. Therefore, the condition of electron beam focusing at the central part and the peripheral parts differs. If the electron beam is focused at the central part, it is not focused at the peripheral parts, resulting in a problem of deteriorated resolution there. In the conventional example shown in FIG. 4, however, the potential of the second member 114 is raised when the electron beam is to be deflected to the peripheral part of the screen. Therefore, the potential difference between the potential of the second member 114 and the accelerating voltage of the accelerating electrode 12 is reduced, and the refractive power of the main lens is weakened. Accordingly, the focusing point of the electron beam is extended in the screen direction, and the electron beam can be focused onto the screen even at the peripheral parts of the screen. From this point as well, it is possible to prevent the resolution at the peripheral parts from being deteriorated. That is to say, it is possible to realize dynamic astigmatism correction and dynamic focus simultaneously.
FIGS. 5A to 5C illustrate another embodiment shown in JP-A-61-250933. In the same way as the embodiment shown in FIGS. 4A to 4C, the focusing electrode is divided into two members 116 and 117. As shown in FIGS. 5B and 5C, vertical and horizontal correction electrodes taking the shape of that plate are so arranged on confronting faces of respective members as to be combined each other, a quadrupole lens being formed. Dynamic voltage Vd superimposed over the focusing voltage Vf is applied to the second member 117 to realize dynamic astigmatism correction and dynamic focus simultaneously.
Further, in JP-A-62-58549, there is shown means of solving a problem of the above described conventional example that application of dynamic voltage lowers the refractive power of the main lens and the converging force applied to outer beams caused by nonaxisymmetric components of the lens, resulting in unsuccessful convergence.
FIGS. 6A to 6C show the structure of an electron gun according to this conventional example. On opposed end faces of the first member 130 and the second member 140 of the focusing electrode as shown in FIGS. 6B and 6C, longitudinal elongated apertures are combined with lateral elongated apertures to form a quadrupole lens in the same way as the conventional example shown in FIG. 4. It is now assumed that outer beam passage holes of a G1 electrode 110 and a G2 electrode 120, outer beam passage holes formed at the G2 electrode side of the first member 130 of the focusing electrode, outer beam passage holes formed on opposed faces of the first member 130 and the second member 140, and outer beam passage holes formed on opposed faces of the second member 140 and the accelerating electrode 150 are located respectively at distance S1, S2, S3 and S4 from the center axis of the electron gun and these distance values are related each other as EQU S1&lt;S2&lt;S3&lt;S4.
In this embodiment, the main lens is so formed as to be axisymmetric, and nonaxisymmetric lenses supplying the converging force to the outer beams are formed on opposed faces of the G2 electrode and the first member. As a result, convergence is not affected even if the refractive power of the main lens is lowered due to a change in dynamic voltage.
The above described prior art has a problem that extremely high precision is demanded in fabrication of components of the electron gun and fabrication of the electron gun. That is to say, when the longitudinal slits are combined with the lateral slits or the longitudinal platelike correction electrodes are combined with the lateral correction electrodes in conventional examples of FIGS. 4A to 4C and FIGS. 5A to 5C, even slight mutual displacement from the desired position causes nonuniform force effected upon the electron beam at the time of astigmatism correction, resulting in a deformed spot on the screen.
Further, in the embodiment of FIGS. 6A to 6C, fabrication of the electron gun becomes further difficult because spaces S1, S2, S3 and S4 of the electron beam passage holes are mutually different. Further, the embodiment of FIGS. 6A to 6C also has a problem that comatic aberration is caused because the outer beam enters the lens slantly.